Method and apparatus for recovering performance of fuel cell stack

ABSTRACT

A method for recovering performance of a fuel cell stack includes 1) a first pulse operation process including i) generating a hydrogen pumping reaction in a cathode by applying a current to the cathode after a supply of air to the cathode stops and ii) maintaining an OCV (open circuit voltage) by again supplying air to the cathode after the hydrogen pumping is performed, 2) a pole substitution process of substituting a pole of the fuel cell stack, and 3) a second pulse operation process including iii) generating the hydrogen pumping reaction in the cathode after the pole substitution, and iv) maintaining the CCV (open circuit voltage) by supplying air to the cathode after the pole substitution after the hydrogen pumping is performed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.14/955,672, filed on Dec. 1, 2015, which claims the benefit of priorityto Korean Patent Application No. 10-2015-0058676, filed on May 18, 2015,the disclosures of which are incorporated in their entirety by referenceherein.

TECHNICAL FIELD

The present disclosure relates to a method and an apparatus forrecovering performance of a fuel cell stack by simultaneously removingoxides and adsorbed impurities on a catalytic surface in an aged fuelcell stack, and more particularly, to a method and an apparatus forrecovering performance of a fuel cell stack by continuously changing ahigh voltage pole of an aged fuel cell stack and alternately generatingan OCV↔hydrogen pumping reaction in a cathode and an anode by acontinuous pulse scheme.

BACKGROUND

A polymer electrolyte membrane fuel cell (PEMFC) for a vehicle may havereduced performance due to deterioration of an electrode (Pt/C, platinumsupported in a carbon support) configuring a membrane electrolyteassembly (MEA) and a membrane after it is operated for a predeterminedtime. In particular, an oxide film (Pt—oxide) formed on a platinumsurface of a cathode having several nano particle size has been known ashaving an effect of hindering reaction O from being adsorbed onto theplatinum surface to reduce an oxygen reduction reaction (ORR) rate ofthe cathode. CO of several ppm included in fuel is chemically adsorbedinto platinum of an anode, which leads to reduced hydrogen oxidationreaction (HOR) efficiency. Further, a local increase in temperatureoccurring during a high power and low humidity operation shrinks a porestructure of the membrane or rearranges an SO⁻ terminal group to cause areduction in ion conductivity. It has been reported that the SO⁻terminal group of a binder causes specific adsorption of an anion into asurface of a platinum catalyst under the low humidity operationconditions to reduce catalyst activation. The SO in air poisons acathode catalyst to reduce the catalyst activation. Since the reductionin performance of the fuel cell stack due to a change in an internalstructure an electrode membrane is due to a reversible deterioration,the performance of the fuel cell stack may be partially recovered.However, research or patents thereon are seldom reported.

The present applicant has tried various methods for improving thereduction in performance of the fuel cell stack due to the reversibledeterioration as described above. As the methods, there are a “methodfor recovering performance of a fuel cell by a hydrogen storage method(Patent Laid-Open Publication No. 10-2014-0017364)” to supply H of hightemperature (70° C.) to a cathode and seal and store both of ananode/cathode under hydrogen atmosphere for 12 hours, a “method forrecovering performance of a fuel cell stack by air braking (PatentApplication No. 10-2013-0146740)” to supply hydrogen to an anode (at thesame time, with a stop of air supply) and then continuously applying aload of 5 to 10 A to induce hydrogen pumping to a cathode, a “method forrecovering performance of a fuel cell stack by a reverse potential pulse(Patent Application. No. 10-2013-0131495)” to substitute a pole and thenapply a high output pulse load in a state in which air is supplied to ananode and hydrogen is supplied to a cathode. All the methods asdescribed above induce the generation of hydrogen to the cathode side toaccelerate a reduction in platinum oxide. However, all the methods donot yet effectively improve the deterioration due to impurities such asCO and SO⁻ adsorbed into the platinum catalyst of the anode among thedeterioration causes of performance of the fuel cell.

SUMMARY

The present disclosure has been made to solve the above-mentionedproblems occurring in the prior art while advantages achieved by theprior art are maintained intact.

An aspect of the present disclosure provides a method and an apparatusfor recovering performance of a fuel cell stack for simultaneouslyremoving platinum oxides of both of an anode and a cathode andimpurities such as CO, S, and SO-adsorbed onto a surface which may notbe solved well by the existing method for recovering performance of afuel cell.

According to an exemplary embodiment of the present disclosure a methodfor recovering performance of a fuel cell stack, includes: 1) a firstpulse operation process including i) generating a hydrogen pumpingreaction in a cathode by applying a current to the cathode after asupply of air to the cathode stops; and ii) maintaining an OCV (opencircuit voltage) by again supplying air to the cathode after thehydrogen pumping is performed; 2) a pole substitution process ofsubstituting a pole of the fuel cell stack; and 3) a second pulseoperation process including iii) generating the hydrogen pumpingreaction in the cathode after the pole substitution by applying acurrent to the cathode after the supply of air to the cathode stopsafter the pole substitution; and iv) maintaining the OCV (open circuitvoltage) by supplying air to the cathode after the pole substitutionafter the hydrogen pumping is performed.

In the pulse operation process of the 1) or 3), the i) and ii) or theiii) and iv) may be repeatedly performed and the i) and ii) or the iii)and iv) each may be repeatedly performed 5 to 10 times.

The 2) pole substitution process and the 3) second pulse operationprocess may be repeatedly performed to alternately remove continuouslyplatinum oxides and impurities remaining at both poles. After the 1) to3) processes are completed in 1 cycle, the 2) and 3) processes may berepeated several times.

The pole substitution process may include supplying hydrogen to an anodeafter the pole substitution and supplying air to the cathode after thepole substitution. The supplied hydrogen may be saturated hydrogen of 65to 75° C., most preferably, saturated hydrogen of 70° C. and thesupplied air may be a saturated air of 65 to 75° C., preferably, asaturated air of 70° C. The air may be supplied along with a coolingwater of 10 to 15° C.

The generating of the hydrogen pumping reaction of the i) or iii) mayinclude continuously applying a load of 0.1 A/cm² for 3 to 5 minutes ina state in which the supply of air stops. The maintaining of the CCV ofthe ii) or iv) may be performed for 0.5 to 1.5 minutes. The 1) firstpulse operation process including the i) and ii) or the 3) second pulseoperation process including the iii) and iv) may be performed 6 timesfor a total of 30 minutes by a pulse scheme.

According to an exemplary embodiment of the present disclosure, anapparatus for recovering performance of a fuel cell stack includes: afuel cell stack having a solid polymer electrolyte membrane between ananode and a cathode; a supply gas flow change mechanism changing a flowof hydrogen and air supplied to the fuel cell stack; and a current flowchange mechanism changing a pole of the fuel cell stack; a gas cuttingoff mechanism cutting off a flow of air to the cathode.

The supply gas flow change mechanism may be a 3-way valve. The currentflow change mechanism may be a contact type relay. The supply gas flowchange mechanism and the current flow change mechanism may beautomatically operated while interlocked with each other.

The apparatus recovering performance of a fuel cell stack according tothe exemplary embodiment of the present disclosure does not have todesorb/reconnect the pipe and dismantle/reconnect the high voltagecable, thereby shortening the recovery time of performance of the fuelcell and saving efforts and costs required to recover the performance.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentdisclosure will be more apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings.

FIG. 1 is a diagram schematically illustrating the existing method forrecovering performance of a fuel cell stack;

FIG. 2 is a diagram schematically illustrating a method for recoveringperformance of a fuel cell stack according to an exemplary embodiment ofthe present disclosure;

FIG. 3 is a graph illustrating a case in which an CCV↔hydrogen pumpingreaction is performed in one electrode six times by a continuous pulsescheme, with respect to time, in the method for recovering performanceof a fuel cell stack according to an exemplary embodiment of the presentdisclosure;

FIG. 4 is a diagram schematically illustrating an apparatus forrecovering performance of a fuel cell stack according to an exemplaryembodiment of the present disclosure;

FIG. 5 is a pair of graphs illustrating recovery rates of performanceand voltage increase quantities for each current density of the fuelcell stack according to the existing method and the exemplary embodimentof the present disclosure;

FIG. 6 is a pair of graphs illustrating a change in a cell voltageaccording to the existing method and the exemplary embodiment of thepresent disclosure; and

FIGS. 7A and 7B are pairs of graphs for comparing primary recovery ratesafter the existing method (a) and the exemplary embodiment (b) of thepresent disclosure are performed only in 1 cycle.

DETAILED DESCRIPTION

A method for recovering performance of a fuel cell stack according to anexemplary embodiment of the present disclosure is to recover theperformance of the fuel cell stack by inducing an OCV (open circuitvoltage, non-load voltage)↔hydrogen pumping reaction to an electrode ina pulse form while continuously changing a high voltage pole of the agedfuel cell stack.

The present disclosure improves the existing method for recoveringperformance of a fuel cell stack focused on a reduction of platinum (Pt)oxides (Pt—OH, Pt—O, etc.) to desorb Pt—SO and Pt—SO⁻ from a cathode anddesorb Pt—SO⁻ and Pt—CO from an anode, such that the present disclosuremay have a 10% or more increase in a recovery rate of performance of afuel cell compared to the existing method (see FIGS. 1 and 2).

As illustrated in FIG. 2, to simultaneously remove platinum oxides andimpurities adsorbed onto a platinum surface of both of the anode and thecathode, the present disclosure discloses what we call a dynamic methodfor recovering performance of a fuel cell stack including a hydrogenpumping process by air braking, a pole substituting process, and a highhumidity cycle operation process.

Describing in more detail the method for recovering performance of afuel cell stack according to the exemplary embodiment of the presentdisclosure, in a fuel cell stack aged due to long-term use, hydrogen issupplied to an anode and air is supplied to the cathode, and a load isapplied in a state in which the air supply stops (air braking) togenerate hydrogen pumping in the cathode. The hydrogen may preferably besaturated hydrogen of 65 to 75° C. and the air may preferably besaturated air of 65 to 75° C. Cooling water of 10 to 15° C. is suppliedas soon as air is supplied, such that condensed water may be generatedon a reaction surface of the fuel cell stack as much as possible. Onlywhen a performance recovery test is performed in the state in which highhumidity conditions are maintained, ions such as SO²⁻ may be desorbed ata hydrogen pumping potential and easily discharged to an outside of acell. Further, for the hydrogen pumping generation, cooling water andhydrogen are continuously supplied and a load of 0.1 A/cm² iscontinuously applied for 3 to 5 minutes in the state in which only thesupply of air stops. In this case, a cell potential reaches about −0.1V.

After the hydrogen pumping reaction is generated within the cathode, airis again supplied into the cathode to maintain the OCV for about 1minute. The OCV↔hydrogen pumping reaction is preferably performed by apulse scheme, preferably, about 5 to 10 times. FIG. 3 illustrates thatthe OCV↔hydrogen pumping reaction is performed, for example, six times.

A mechanism of removing the impurities adsorbed onto the platinumsurface by maintaining the hydrogen pumping reaction and the OCVconditions in the cathode is shown in the following Table 1.

TABLE 1 Mechanism of recovering performance of an aged fuel cell stackby ‘OCV ↔ hydrogen pumping reaction’ (reaction mechanism in cathodeunder normal potential condition) Impurity Chemical Additional SpeciesReaction Mechanism Explanation Pt—OH/O Pt—OH + H⁺ + e⁻ → Pt + Reduced atH₂O hydrogen Pt—O + 2H⁺ + 2e⁻ → Pt + pumping potential H₂O Pt—SO_(x)Pt—SO_(x) → Pt—S^(o) _(ads) Reduction into sulfur at hydrogen pumpingpotential (about 0.05-0.1 V vs. SHE) Pt—S Pt—S + 4H₂O → Pt—SO₄ ²⁻ +Hydrated into 8H⁺ + 6e⁻ sulfate form under OCV condition (0.9- 1.3 V)Pt—SO₄ ²⁻ Pt—SO₄ ²⁻ or Pt—(H)SO₄ ⁻ → Desorbed at Pt hydrogen pumpingpotential at which many droplets are present and discharged to outsideof cell

The platinum oxides in the cathode and the sulfur oxide adsorbed ontothe platinum surface may be removed by the OCV↔hydrogen pumpingreaction. After the pulse reaction, a pole is substituted(anode↔cathode) to supply air to the anode before the pole substitution(cathode after the pole substitution) and hydrogen to the cathode beforethe pole substitution (anode after the pole substitution). In this case,according to the method and the apparatus for recovering performance ofa fuel cell stack according to the exemplary embodiment of the presentdisclosure, a supply gas flow changing mechanism such as a 3-way valvefor changing a flow of air and hydrogen without manually desorbing apipe (pipe through which saturated air and saturated hydrogen aresupplied) connected to the anode and the cathode every time is provided(see FIG. 4). Further, a high voltage cable (+)/(−) connected to thestack also needs to be dismantled depending on the changed and suppliedreaction gas, replaced, and again connected. The apparatus forrecovering performance of a fuel cell stack according to the exemplaryembodiment of the present disclosure may include a current flow changemechanism such as a contact type relay to interlock a reaction gas flowsupplied to the stack with a high voltage terminal pole, therebyautomatically changing the gas flow (see FIG. 4).

According to the apparatus for recovering performance of a fuel cellstack the exemplary embodiment of the present disclosure, the anode andthe cathode are substituted with each other. After the pole issubstituted, the saturated air of 65 to 75° C. and the cooling water of10 to 15° C. are supplied to a cathode and after the pole substitution,the saturated hydrogen of 6 to 75° C. is supplied to the anode and thena load is applied to the cathode in the state in which the supply of airstops to generate the hydrogen pumping reaction in the cathode for about2 to 10 minutes, preferably, 3 to 5 minutes. Next, air is again suppliedto the cathode to maintain the OCV for about 0.5 to 1.5 minutes,preferably, 1 minute. Similar to the cathode before the polesubstitution, the OCV↔hydrogen pumping reaction is performed about 5 to10 times even in the cathode after the pole substitution by the pulsescheme, preferably, 6 times as illustrated in FIG. 3.

The mechanism of removing impurities adsorbed onto the platinum surfaceby maintaining the hydrogen pumping reaction and the OCV conditions inthe cathode (anode under the normal potential conditions) after the poleis first substituted is shown in the following Table 2.

TABLE 2 Mechanism of recovering performance of an aged fuel cell stackby ‘OCV ↔ hydrogen pumping reaction’ (reaction mechanism in an anodeunder a normal potential condition) Impurity Chemical Additional SpeciesReaction Mechanism Explanation Pt—CO Pt—CO + Pt—O → 2Pt + CO₂ Desorbedby electrochemical reaction with oxygen supplied at the time of polesubstitution Pt—CO Pt—CO + OH_(ads) → Pt + CO₂ + Electrochemically H⁺ +e⁻ desorb CO while reaching OCV at the time of pole substitution Pt—SO₃⁻ Pt—SO₃ ⁻ → Pt Desorbed at hydrogen pumping potential at which manydroplets are present and discharged to outside of cell

If the OCV↔hydrogen pumping reaction is performed in 1 cycle for 30minutes in the cathode under the normal potential conditions and for 30minutes in the anode under the normal potential conditions and then theabove process is performed in a total of 2 to 5 cycles (total requiredtime: 2 to 5 hours), it could be appreciated that the recovery rate is a10% increase in the recovery rate of perfolmance of the fuel cellrelative to the existing method.

Further, the apparatus for recovering performance of a fuel cell stackaccording to the exemplary embodiment of the present disclosureautomatically substitutes the pole without artificially changing theposition of the pipe/cable and continuously performs the hydrogenpumping to the anode and cathode to recover the catalyst activation ofboth electrodes, thereby considerably shortening the performancerecovery time and cost.

Hereinafter, examples of the present disclosure will be described inmore detail. However, these examples are to describe in more detail thepresent disclosure and the scope of the present disclosure is notlimited thereto.

EXAMPLE 1 Recovery Rate and Voltage Increase Quantities of Fuel CellBetween the Existing Method and the Method of the Present Disclosure

To determine how much the recovery rate is improved by comparing themethod for recovering performance of a fuel cell stack according to thepresent disclosure with the existing method, the performance recoveryfrequency was performed 0 to 5 times and the result was shown in a graphof FIG. 5.

The performance recovery frequency of 1 time defines, a 1 time, arecovery cycle of about 1 hour which is a sum of the case in which theOCV↔hydrogen pumping reaction is performed 6 times (total required time:30 minutes) in the cathode by the pulse scheme under the normalpotential conditions with the case in which the OCV↔hydrogen pumpingreaction is also performed 6 times (total required time: 30 minutes) inthe anode by the pulse scheme under the normal potential conditionsafter the pole substitution, in the fuel cell stack aged due to the longterm use.

As the existing method, an air braking method (after the supply ofhydrogen to the anode (at the same time, a stop of air supply), a methodfor continuously applying a load of 5 to 10 A to induce the hydrogenpumping to the cathode) was used to reduce the platinum oxides only inthe cathode.

The aged fuel cell stack (@0.6 A/cm² stack) used in the experiment isthe aged stack for a vehicle after the vehicle drives a distance of6,500 km and the performance recovery was performed on upper/lowermodules (Upper end of the method of the present disclosure vs. lower endof the existing method) and as a result of performing the method of thepresent disclosure and the existing method 1 to 5 times at a currentdensity of 0.6 A/cm², it could be appreciated that the recovery rate ofthe case using the method of the present disclosure is 40.9% and therecovery rate of the case using the existing method is 29.3%, andtherefore the recovery rate is increased as much as about 10%. Theperformance recovery rate indicates a voltage value recovered after therecovery as a percentage when a voltage drop of the aged stack isassumed to be as 100 on the basis of the performance at the time of thebeginning of life (BOL). Further, it could be appreciated that about 30%is recovered only by performing the method of the present disclosure in1 cycle under the same conditions. In this case, it could be appreciatedthat the time taken to reach a recovery rate of 30% is greatly shortenedas compared to the existing method. The comparison of the primaryrecovery rate is disclosed in more detail in the following Example 2.

Further, as illustrated in the right graph of FIG. 5, it could beappreciated that in the case of using the method of the presentdisclosure, an average voltage of a plurality of cells is uniformlyincreased for each current density. Here, it could be appreciated thatthe current density is increased by 17 mV at the current density of0.6A/cm², which is more increased than 12 mV which is a voltage increaseamount in the case of using the existing method under the sameconditions.

Further, by reviewing the cell average voltages before the recovery ofeach of the plurality of cells, after 3 cycle recovery, and after 5cycle recovery at the current density of 0.6 A/cm², it could beappreciated that the average voltage of each cell is high and adeviation in the average voltage between the cells is small, compared tothe case of using the existing recovery method (see FIG. 6).

EXAMPLE 2 Comparing Recovery Rates when a Method for RecoveringPerformance of Fuel Cell Stack of the Present Disclosure is PerformedOnly in 1 Cycle

The Example 2 is the same as the above Example 1 except that theperformance of the upper end module of the stack for a vehicle isrecovered by the existing method and the performance of the lower endmodule is recovered by the method of the present disclosure andtherefore comparing the method of the present disclosure with theexisting method, each performance recovery method was performed only in1 cycle.

The 1 cycle of the performance recovery method of the present disclosuremeans the primary performance recovery of performing the OCV↔hydrogenpumping reaction 6 times for 30 minutes in one electrode and thenperforming the OCV↔hydrogen pumping reaction for 30 minutes in the otherelectrode by substituting the pole, in which the total required time wasabout 1 hour.

The time required in 1 cycle of the existing method described in theabove Example 1 was 3 hours.

As illustrated in FIGS. 7A and 7B, the primary recovery rate of the case(7A) of using the existing method was about 19% and the primary recoveryrate of the case (7B) using the method of the present disclosure wasabout 31% and therefore it could be appreciated that the recovery rateis greatly improved in the case of using the method of the presentdisclosure.

As described above, according to exemplary embodiments of the presentdisclosure, the method for recovering performance of a fuel cell stackaccording to the exemplary embodiments of the present disclosure maysimultaneously remove the oxides of the platinum surface occurringduring the use of the fuel cell stack and the impurities such as CO andsulfur oxides introduced from the outside of the stack to increase therecovery rate and the efficiency from that of the existing method forrecovering performance of a fuel cell, thereby expanding the life of thefuel cell.

Although the preferred embodiments and application examples of thepresent disclosure have been disclosed for illustrative purposes, thoseskilled in the art will appreciate that the present disclosure is notlimited to specific embodiments and application examples and variousmodifications, additions and substitutions are possible, withoutdeparting from the scope and spirit of the disclosure as disclosed inthe accompanying claims. Accordingly, such modifications, additions andsubstitutions should also be understood to fall within the scope of thepresent disclosure.

What is claimed is:
 1. An apparatus for recovering performance of a fuelcell stack, comprising: a fuel cell stack having a solid polymerelectrolyte membrane between an anode and a cathode; a supply gas flowchange mechanism for changing a flow of hydrogen and air supplied to thefuel cell stack; a current flow change mechanism for changing a pole ofthe fuel cell stack; a gas cut off mechanism for cutting off a flow ofair to the cathode.
 2. The apparatus according to claim 1, wherein thesupply gas flow change mechanism is a 3-way valve.
 3. The apparatusaccording to claim 1, wherein the current flow change mechanism is acontact type relay.
 4. The apparatus according to claim 1, wherein thesupply gas flow change mechanism and the current flow change mechanismare automatically operated while interlocked with each other.